Owing to its importance in hydrogenation catalysis, the interaction of hydrogen with metal surfaces has been extensively studied in recent years, both experimentally and theoretically. A detailed knowledge of the underlying reaction mechanisms, in particular those leading to dissociation of H2, can be useful, e.g., to design better catalysts. Most experimental approaches are based on measuring sticking coefficients under well-defined incidence conditions. On the other hand, diffraction experiments with H2 beams have also been proposed as a promising tool to gauge the molecule-surface interaction, i.e., the potential energy surface (PES) that ultimately controls dissociation. One important difference between H2 diffraction experiments and the more traditional He ones is the eventual competition with dissociation [1]. In addition, for reactive metal surfaces, reflectivity is very small, which makes much more challenging the observation of H2 diffraction. In spite of this, the excellent agreement found very recently between experimental and theoretical results from state-of-the-art PES's provides a new and strong support to the use of diffraction experiments [2].
We have measured in-plane and out-of-plane diffraction of H2 molecular beams scattered from Pt(111) at incident energies from 25 to 150 meV. Out-of-plane diffraction was found to be very intense for most of the incident conditions used. A Debye–Waller-type attenuation of diffraction intensities as a function of surface temperature was verified in the whole energy range studied. This allowed extrapolation of absolute diffraction intensities to 0 K, which agreed very well with those obtained from six-dimensional quantum dynamics calculations using the Born-Oppenheimer approximation [2]. We also performed closed-coupling calculations for different surfaces and model potentials. They show that the pronounced out-of-plane diffraction observed can be easily explained in terms of energy transferred arguments, and that it is actually a general feature of three-dimensional scattering problems.
[1] D. Farías and K.H. Rieder, Rep. Prog. Phys. 61, 1575 (1998).
[2] P. Nieto, E. Pijper, D. Barredo, G. Laurent, R.A. Olsen, E.J. Baerends, G.J. Kroes, and D. Farías, Science 312, 86 (2006).
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